"Decrypted" is the term given to the upgraded uncroaked (or "popped again") Wanda creates after attuning to the Arkenpliers. Decrypt is used as a verb (he was decrypted), an adjective (a decrypted unit), and a noun (decrypted retain their stats), similar to the usage of uncroaked. Decrypted superficially appear to be alive, lacking the physical evidence of the trauma that croaked them.
Unlike uncroaked units, decrypted units do not decay.Erf-b1-p145
Decrypted are loyal to the tool wielder, regardless of past allegiances, but still remember their past loyalties, any information they knew in the past, and loves. Erf-b1.5-p033
Decrypted units have zero upkeep cost and retain more of their pre-death strength, if not full strength, than uncroaked.Erf-b1-p145
Decrypted Natural Allies are units of the side of the wielder of the Arkempliers, and don't count as a separate tribe, so if all members of a Natural Allies tribe are croaked, not even Decrypting them can allow to build more units of their former tribe.Erf-b1.5-p045
The unit is enslaved to Wanda and may suffer a major mental attitude adjustment (e.g. "You were correct, of course. Royalty is obsolete."Erf-b1-p145) possibly due to a change in its Natural Thinkamancy or a choice made by Wanda at the time of decryption; otherwise, the Unit retains all of its previous stats, abilities, and levels, but it may suffer a demotion if it was of a high rank and that position is already occupied by another Unit.
As with uncroaking, uniforms and insignia are changed when the uncroaked join their new side. Decrypted seem to gain Wanda's insignia Erf-b1-p144 rather than the insignia of their chief warlord Erf-b1-p116, but this is Wanda's choice for units she creates Erf-b1.5-p026 rather than a feature of decryption itself. Units like Wiener-Rammers with no uniform may suffer a change of color in skin or fur.Erf-b1-p146
Wanda may be able to see through her units eyes, and if not, they will tell her anything relevant that happened to them. Erf-b1.5-p044
Decrypted still are able to gain experience.
As they have zero upkeep, it is unlikely that any food pops for them, and thus unlikely that they need to eat and drink. It is unclear whether they can eat and drink.
Assuming that Wanda managed to decrypt at least one Gobwin, the fact that Stanley hasn't been able to obtain any new Gobwin allies suggests that decrypted natural allies can't produce new members of their former unit type.
It is unknown as of yet exactly how the Arkentools do what they do. It seems that they give Wanda an advanced form of Croakamancy, but it is a theory that it makes a combo-caster. Allowing Wanda to use a Croakamancy/Healomancy/Turnamancy spell to make the decrypted.
Real World References
Decryption, in the real world, is used as the opposite of encryption, that is protecting data by turning it into a seemingly random sequence of symbols by means of a cypher and a key. Decryption is the opposite process, which takes useless gibberish and turns it into useful information, paralleling the process of taking a useless corpse and turning it into a useful creature. The same term is sometimes also used for removing desired content from protected sources (such as DVDs, music files and books). In this case, Wanda might have gained the ability to effectively remove the "DRM" from enemy units, allowing her to raise them without penalties and under her control. Alternately, the process of updating the MMORPG DDO contains a decrypting step, possibly suggesting the process is just an update to the units rules.
This makes the term a double pun, the 1st one being on "getting the unit out of a crypt".